The disclosed technology relates generally to methods and tools for micro-transfer-printing. It is often difficult to pick up and place ultra-thin and/or small devices using this technology. Micro transfer printing permits the selection and application of these ultra-thin, fragile, and/or small devices without causing damage to the devices themselves.
Micro-transfer-printing allows for deterministically assembling and integrating arrays of micro-scale, high-performance devices onto non-native substrates. In its simplest embodiment, micro-transfer-printing is analogous to using a rubber stamp to transfer liquid-based inks from an ink-pad onto paper. However, in micro-transfer-printing the “inks” are composed of high-performance solid-state semiconductor devices and the “paper” can be substrates, including plastics and other semiconductors. The micro-transfer-printing process leverages engineered elastomer stamps coupled with high-precision motion-controlled print-heads to selectively pick-up and print large arrays of micro-scale devices onto non-native destination substrates.
Adhesion between the elastomer transfer device and the printable element can be selectively tuned by varying the speed of the print-head. This rate-dependent adhesion is a consequence of the viscoelastic nature of the elastomer used to construct the transfer device. When the transfer device is moved quickly away from a bonded interface, the adhesion is large enough to “pick” the printable elements away from their native substrates, and conversely, when the transfer device is moved slowly away from a bonded interface the adhesion is low enough to “let go” or “print” the element onto a foreign surface. This process may be performed in massively parallel operations in which the stamps can transfer, for example, hundreds to thousands of discrete structures in a single pick-up and print operation.
Micro transfer printing also enables parallel assembly of high-performance semiconductor devices onto virtually any substrate material, including glass, plastics, metals, or other semiconductors. The substrates may be flexible, thereby permitting the production of flexible electronic devices. Flexible substrates may be integrated in a large number of configurations, including configurations not possible with brittle silicon-based electronic devices. Additionally, plastic substrates, for example, are mechanically rugged and may be used to provide electronic devices that are less susceptible to damage or electronic performance degradation caused by mechanical stress. Thus, these materials may be used to fabricate electronic devices by continuous, high-speed, printing techniques capable of generating electronic devices over large substrate areas at low cost (e.g., roll to roll manufacturing).
Moreover, these micro transfer printing techniques can print semiconductor devices at temperatures compatible with assembly on plastic polymer substrates. In addition, semiconductor materials may be printed onto large areas of substrates thereby enabling continuous, high speed printing of complex integrated electrical circuits over large substrate areas. Moreover, fully flexible electronic devices with good electronic performance in flexed or deformed device orientations may be provided to enable a wide range of flexible electronic devices.
Micro-structured stamps may be used to pick up micro devices, transport the micro devices to the destination, and print the micro devices onto a destination substrate. The transfer device (e.g., micro-structured stamp) can be created using various materials. Posts on the transfer device can be generated such that they pick up material from a pick-able object and then print the material to the target substrate. The posts can be generated in an array fashion and can be a range of heights depending on the size of the printable material. Compression (in the z direction) of the transfer device can be used to fully laminate the array of printable objects to the posts of the transfer device. Additionally, compression can be used to allow for a critical velocity to be reached by increasing the distance the stamp is moved at a set acceleration based on the equation v2=2ad.
However, compression of the transfer device poses several issues. Among other things, there is a possibility of sagging between posts. This sag allows for unwanted materials to be picked up from the source substrate. As the span between adjacent posts is increased, the risk of sag causing problems increases. Additionally, there is a crowning effect that can be noted at the edge of the transfer device bulk material that is caused by the coefficient of thermal expansion (CTE) mismatch between the bulk material and the hard plate interface (e.g., glass) as shown, for example, in FIG. 22. Thus, there is a need for techniques that minimize or eliminate at least these issues and increase bonding when devices are printed.
Transfer printing with a visco-elastic stamp material requires a high-velocity separation between stamp and source material to “pick” chips. Typical applications use approximately 1 g of acceleration to accomplish the chip or die “pick” process step. However, the velocity at separation occurs at small distances (e.g., tens of microns or less) dependent on the compression of the stamp at lamination. Thus, there is a need for greater acceleration to create higher separation velocities at small distances that in turn increases the adhesion between the stamp and source.